In real-time-critical distributed systems, the network data streams comprising a set of network frames have end-to-end constraints to be complied with between the production of the data from a sender entity and the consumption of the data by a receiver entity. During the design of such systems, these constraints must be complied with and the network resources must be dimensioned correctly with respect to the applicational needs. According to the architecture used to implement the transmission of the data streams, the dimensioning may pertain:                to the duration of the send slots allocated to the various entities of the distributed system, when the network topology requires the deployment of arbitration of entity access to the communication network, such as for example on a data bus managed via an access policy of Time Division Multiple Access (TDMA) type; or        to the size of the queues within the network switches when the operation of the system relies on a switched network, such as for example in the case of full-duplex switched Ethernet.        
However, to design real-time-critical distributed systems which contain at least three network nodes with the existing architectures, the network resources are required to be overdimensioned, whatever the dimensioning already carried out in order to satisfy the end-to-end constraints, the overdimensioning increasing rapidly with the number of nodes. Indeed, the dimensioning of the send slots, when arbitration in relation to network access by an entity is required, relies on the assumption of a common global clock pooled between the various entities so that access to the network resources can be correctly shared. However, each entity having its own clock, the common global clock is approximated by defining an acceptable bound, called synchronization precision, for the various clocks of the entities. The more significant the precision, the smaller the uncertainties in the instants of data exchange and therefore the better is the use of the bandwidth. As the number of entities grows, the precision decreases and the utilization of the bandwidth decreases, hence requiring the overdimensioning of the network capabilities. In the case of the dimensioning of the queues of network switches, there is a very large number of possible states of these queues of the various switches of the network while satisfying compliance with the end-to-end constraints for the applicational data streams as a whole. The solution in relation to the required size of these queues is either greatly pessimistic, hence requiring significant overdimensioning of the resources, or else may be exact but on relatively small sizes of system. Thus, the capabilities of a communication network that are required for the execution of critical real-time distributed systems must be overdimensioned, thus increasing the cost of implementing such systems.
Moreover, independently of the retransmission policy used within a switch, whether it be in direct or deferred mode for example, current switches do not consider the applicational content of the network frames. Thus, the applicational part of a network frame is viewed as a black box whose content is completely retransmitted on a set of ports of the network switch, as a function of the fact that it is a frame of unicast, multicast or broadcast type. However, the entire applicational content of a frame might not be relevant to the whole set of addressee entities of a frame of multicast or broadcast type. For example, it is possible that the addressees of network frames within a network domain such as a set of entities bus-connected on a port of the switch may use only a part of the applicational content of the frames received by the switch. Moreover, each part of the applicational content of a network frame necessary for an entity may be different from one entity to the next. It is also possible that part of the applicational content of a frame may not be used by any of the addressee entities of a network domain, but on the contrary by addressee entities of another network domain, thereby justifying in the current solutions the inclusion of data which is unnecessary in respect of a network domain, this frame part then occupying network bandwidth unnecessarily. Consequently, the actual effectiveness of the network transmission, which is the ratio of the quantity of applicational data transmitted to the size of the frame, is therefore reduced. Moreover, the latency in dispatching such a frame is increased in a likewise unnecessary manner, this being particularly accentuated if the sum of the data actually necessary for the network domain constructed is reduced.
Currently, to ensure the fact that the data present in a frame are completely necessary for a network domain, a set of network frames must be constructed, each containing solely the data required by the network domain considered. This requires that the number of frames sent be increased and consequently that their size be decreased and also that the number of preambles and postambles of the frames be increased. Likewise, the minimum number of periods of inactivity between two frames, said periods being called “interframe gap”, is also increased. For all these reasons, the use of the network bandwidth is therefore decreased.
Thus, the current procedures for retransmission of data by a network switch entail under-use of the capabilities of the networks connected to the switches.
There thus exists the need for a solution which makes it possible at one and the same time to satisfy compliance with end-to-end constraints for the dissemination of data while best utilizing the capabilities of a distributed real-time network.
The present invention deals with this need.